Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers so as to define a p-n junction. When a bias is applied across the p-n junction, holes and electrons are injected into the active layer where they recombine to generate light in a process called injection electroluminescence. Light may be emitted from the active layer through all surfaces of the LED.
As most LEDs are nearly monochromatic light sources that appear to emit light having a single color, light emitting devices or lamps including multiple LEDs that can emit light of different colors have been employed to produce white light. In these devices, the different colors of light emitted by the individual LEDs combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white.
As an alternative to combining individual LEDs to produce light emitting devices having a particular light emission spectrum, luminescent materials, or phosphors, may be used to control the color of light emitted from LEDs. A phosphor may absorb a portion of the light emitted from an LED at a given wavelength and re-emit the light at different wavelength via the principle of photoluminescence. The conversion of light having a shorter wavelength (or higher frequency) to light having a longer wavelength (or lower frequency) may be referred to as down conversion. For example, a down-converting phosphor may be combined with a blue LED to convert some of the blue wavelengths to yellow wavelengths in order to generate white light.
A widely used phosphor for white light generation is yttrium aluminum garnet (YAG) doped with cerium (Ce), i.e., Y3-xCexAl5O12 or YAG:Ce. This yellow phosphor may be used in combination with a blue LED to produce white light. Compared to other phosphors based on silicates and sulfides, for example, YAG:Ce has a relatively high absorption efficiency of blue excitation radiation, a high quantum efficiency (greater than 90%), good stability in high temperature and/or high humidity environments, and a broad emission spectrum.
In some cases, a red phosphor is added to a light emitting device including a blue LED and a YAG:Ce phosphor in order to further shift the emitted light into the desired neutral white color bins (e.g., E3 to E6). Red phosphors are generally less efficient emitters than YAG:Ce, however, and thus the luminous efficiency of the light emitting device may be decreased when a red phosphor is used. It is therefore of interest to find other ways to shift the light emission of yellow phosphors to longer wavelengths.
Several current approaches to shifting the YAG:Ce emission to longer wavelengths involve chemical substitution (doping and/or co-doping) of yttrium, aluminum, and/or oxygen of the garnet lattice structure with other atoms. For example, gadolinium (Gd) or terbium (Tb) may be substituted for Y; chromium (Cr) may be substituted for Al; silicon (Si) and magnesium (Mg) may be substituted for Al; praseodymium (Pr) and Cr may be simultaneously substituted for Y and Al, respectively; and Si and nitrogen (N) may be simultaneously substituted for Al and O, respectively.
However, the resulting phosphor may exhibit a lower luminescence efficiency compared to mixtures of yellow and red phosphors, or the shift in wavelength may be accompanied by a decrease in the luminescence quenching temperature (i.e., the phosphor emission or conversion efficiency may decrease with increasing temperature). This means that the operation of the LED device may be impaired at elevated temperatures (e.g., 30-45° C. and above), thereby causing a substantial loss of emission intensity.